Lubricating oils are commonly used in compressors to reduce friction, enhance sealing, and remove heat in cylinders and crankcase assemblies. Cylinder lubricating oils are formulated with one or more inhibitors to help prevent ethylene polymerization inside the compressor and elsewhere in the process piping. The oil lubricates a cylinder assembly that may have sealing rings moving against a stationary surface or stationary sealing rings bearing on a moving surface. Due to loads developed at high pressure, these components are usually metallic. Typical base fluids for these lubricating oils include mineral oils and a variety of synthetic oils. Additives are included for corrosion protection, metal passivation, antiwear, oxidative stability, viscosity modification, lubricity, and other purposes. Because of the demanding applications, much research has been done to improve lubricating oils.
For example, U.S. Pat. No. 4,116,874 discloses a compressor oil comprising a mineral oil, an aromatic amine, a triester of dithiophosphoric acid, and an alkyl phenol selected from a list of six hindered phenols, none of which is a 4-hydroxybenzoate.
U.S. Pat. No. 5,156,759 discloses a high-temperature compressor oil comprising a poly-α-olefin, an ester oil, a rust inhibitor, and an antioxidant comprising octylbutylphenylamine and a t-butyl phenolic antioxidant. Fourteen hindered phenols are mentioned in the description but none of them is a 4-hydroxybenzoate.
U.S. Pat. No. 5,798,319 teaches a synthetic oil and various additives including viscosity modifiers, dispersants, detergents, anti-wear agents, friction modifiers, corrosion inhibitors, rust inhibitors, demulsifiers, antifoam agents, and antioxidants. Hindered phenols are recommended antioxidants, but no examples are given, and a 4-hydroxybenzoate is not mentioned.
U.S. Pat. No. 6,329,327 discloses a lubricant containing an organic molybdenum compound. Other additives are mentioned including phosphites and hindered phenols. More than forty-five hindered phenols are mentioned, but none of them is a 4-hydroxybenzoate.
U.S. Pat. No. 6,656,887 teaches a lubricating oil with a phosphate. Hindered phenol antioxidants are mentioned and more than twenty examples are given. None of them is a 4-hydroxybenzoate.
U.S. Pat. No. 6,756,346 discloses a lubricating oil composition with an antiwear compound and a rust preventer. More than thirty-five hindered phenols are mentioned as possible antioxidant additives, but none of them is a 4-hydroxybenzoate.
U.S. Pat. No. 6,756,346 discloses a lubricating oil composition containing an antiwear additive and a rust preventing additive. Hindered phenols are mentioned as antioxidants and a long list is given as examples. None is a 4-hydroxybenzoate.
U.S. Pat. No. 5,578,557 discloses a compressor oil suitable for the manufacture of polyethylene. The oil comprises a base fluid such as white oil, a sarcosine derivative for friction protection, an imidazoline corrosion inhibitor, and an amine phosphate. Hindered phenols are identified as possible antioxidant additives, but no 4-hydroxybenzoates are mentioned. Corrosion and wear testing is reported, but there is no information on the ethylene polymerization.
According to U.S. Pat. No. 6,172,014, “the escalating requirements for lubricants used in compression cylinders associated with the dynamic sealing of ethylene . . . is considered one of the most demanding aspects of high-pressure manufacture of polyethylene.” A phosphite in combination with a second antioxidant is recommended. Hindered phenols are listed as suitable second antioxidants. Different types of suitable hindered phenols are described, but none is a 4-hydroxybenzoate.
Despite the considerable research and myriad of formulations tested, there remains a need for improved lubricating oils for use in α-olefin compressors. Hindered phenol antioxidants, especially hindered phenols with electron donating groups such as alkyl or alkoxy in the para position, help to minimize oxidation of the oil. However, using antioxidants has tradeoffs. Usually, traces of the lubricating oil enter the high-pressure polymerization reactor along with the α-olefin, so the lubricant's antioxidant can interfere with the polymerization or cause erratic behavior, especially during a process start-up. Thus, the need remains for a lubricating oil that has sufficient oxidative stability but also has a limited impact on the α-olefin polymerization.